More support for reading data files that live in zipfiles.

This is a continuation of afb2e073fd6f37addef79e3e1e7df7b09d9c6fb7.
When I made that change, I didn't notice this os.path.exists() call as
well.

Test plan:
Fingers crossed.  This turns out to be very difficult to test on
localhost due to dev-appserver not dealing well with .pyc files inside
zipfiles.

But I did run
   % cd deploy; python -c 'import deploy, os; os.chdir(".."); deploy.compress_py(False)'
   % python
   >>> import third_party.babel.localedata as ld
   >>> ld._dirname = 'genfiles/third_party.zip/third_party/babel/localedata'
   >>> ld.exists('en')
   True
   >>> ld.exists('enn')
   False
so hopefully it's ok.